At least one embodiment of the invention relates to an antenna diversity system for relatively broadband broadcast reception in vehicles such as motor vehicles. The term Broad band such as mobile broadband can be used to describe various types of wireless high-speed internet access through a portable modem, telephone or other device. Examples of different broadband network standards that may be used, include EV-DO, WiMAX, UMTS/HSPA, or some portable satellite-based systems.
In this case, multi-path propagation leads to narrowing of the bandwidth of the channel from the transmitter antenna to the mobile receiver if the path differences of the electromagnetic wave bundles that arrive at the reception location are not small enough to be ignored. Therefore, there is a dependence in frequency at a reception location that is similar to the one observed at a fixed frequency over the driving path. This dependence is illustrated in FIG. 2a, in a wave field with Rayleigh distribution, in a two-dimensional representation.
Particularly in the case of relatively broadband broadcast reception, whose channel width is greater than the bandwidth of the transmission channel as a result of multi-path reception, this phenomenon leads to interference that is known from television reception in vehicles. FIG. 2c shows the level distribution at a location, plotted above the frequency, and shows that a reception minimum exists at a frequency deviation of about 5.5 MHz from the video carrier, for example. In the case of a diversity system according to the scanning method, an antenna that is selected for good reception of the video carrier therefore cannot receive the audio carrier equally well. For this reason, the signals of a multi-antenna system are separated, in the European patent EP 0521 123 B1 which is also published in corresponding U.S. Pat. No. 5,313,660, to Lindenmeier et al which issued on May 17, 1994 the disclosure of which is hereby incorporated herein by reference in its entirety. This patent disclosure shows the signals of the multi-antenna system are separated by means of separate use of a diversity system for video and audio reception, in each instance.
Nevertheless, the disadvantage remains that the video signal components that lie far away from the video carrier are reproduced only deficiently. In the case of digitally modulated, relatively broadband transmission methods, in particular—such as the DVBT method (Digital Video Broadcasting Terrestrial) and the DAB method (Digital Audio Broadcasting)—the loss due to non-detectable symbols at elevated bit error rates frequently has such an effect that the broadcast connection breaks off.
Thus, one benefit of the invention is that it creates a particularly efficient antenna diversity system, which avoids the disadvantages connected with an overly low bandwidth of the transmission channel, to a great extent.